Some crystalline aluminosilicates, or zeolites, are useful as adsorbents in separating certain hydrocarbon compounds from mixtures containing those compounds. In particular, zeolites are widely used for selective separation of paraxylene from mixtures with other C.sub.8 aromatic compounds such as metaxylene, orthoxylene, ethylbenzene. For example, U.S. Pat. Nos. 3,636,121; 3,686,342; 3,686,343; 3,835,043; 3,855,333; 3,878,127; 3,894,108; 3,903,187 and 4,265,788 are all directed toward methods of separating paraxylene from mixtures with various hydrocarbons or of selectively obtaining paraxylene and ethylbenzene from a mixture containing other components, using various types of zeolites as adsorbents.
Paraxylene is a commercially important aromatic hydrocarbon since its use in the manufacture of terephthalic acid is a critical step in the subsequent production of various fibers such as Dacron.
This invention relates generally to a process for separating paraxylene from mixtures of other C.sub.8 aromatics by selectively adsorbing the paraxylene on a faujasite zeolite, particularly sodium or potassium exchanged Y zeolites. The paraxylene is desorbed either through the use of cyclic hydrocarbonaceous molecules containing a nitrogen atom either alone or in combination with other desorbents such as benzene or paradiethylbenzene.
It is known that potassium-substituted Type Y zeolites having the faujasite structure selectively adsorb ethylbenzene from mixtures comprising ethylbenzene, metaxylene and orthoxylene using toluene as a desorbent. U.S. Pat. No. 3,998,901 teaches that ethylbenzene can be separated from xylene isomers using a Type Y zeolite substituted with Sr and K wherein ethane or lower gases or toluene is used as desorbent. According to U.S. Pat. No. 3,943,182, desorbents other than toluene, such as diethylbenzene or benzene, selectively separate ethylbenzene from a mixture containing ethylbenzene and at least one xylene isomer using a Type X zeolite. It is also disclosed that selectivity for ethylbenzene over its isomers decreases as the silica to alumina ratio in the zeolite is increased above 3.0 (i.e., using a Type Y zeolite).
U.S. Pat. No. 3,943,182 further teaches that the presence of water in the zeolite in amounts of 0.02 to 2.5% by weight measured by loss on ignition at 500.degree. C. optimizes selectivity for ethylbenzene. Other patents disclose that certain compounds will modify the adsorbent characteristics of zeolites when contacted therewith. For example, in the context of aromatic isomer separation, U.S. Pat. No. 3,698,157 discloses that an organic radical-substituted silane modifies the characteristics of a selected zeolite in the separation of C.sub.8 aromatic isomers. In U.S. Pat. No. 3,734,974 it is taught that faster exchange rates and reduced orthoxylene and metaxylene tailing are accomplished by adding small amounts of water to a particular adsorbent. Moreover, U.S. Pat. No. 3,855,333 is directed to use, as an adsorbent, of a zeolite containing 0.1 to 8.0% by weight of an alcohol to obtain increased selectivity of the zeolite for adsorption of paraxylene.
U.S. Pat. No. 4,283,587 to Rosback, issued Aug. 11, 1981, discloses a process for enhancing the selectivity of various substituted (usually potassium or barium substituted) Type X or Y zeolites for paraxylene over other C.sub.8 -aromatics. By preloading the zeolite with an alkyl amine hydrochloride or alkyl amine, the activity of the desorbents (particularly that of paradiethylbenzene) for removal of the adsorbed aromatic from the zeolite is improved as well.
U.S. Pat. No. 4,351,981 to Smolin, issued Sept. 28, 1982, discloses a process for the separation of paraxylene from mixtures of C.sub.8 -aromatics by contacting the mixture with either a pyridine-modified LiX or NaX zeolite. The process involves preferably preloading the zeolite with an amount of pyridine in an amount of about 10% to 60% of the total adsorptive capacity of the zeolite, e.g., preferably 3-6 wt.% based on the zeolite weight. The process is said to adsorb the C.sub.8 -aromatics to the exclusion of paraxylene. Consequently, it is said to be desirable to minimize the amount of the C.sub.8 -aromatics other than paraxylene by, e.g., distillation, prior to treating the stream with the modified zeolites. The pyridine may be used with another desorbent which preferably is toluene.
Similarly, U.S. Pat. No. 4,393,266 to Smolin, issued July 17, 1983 (the continuation-in-part of Smolin '981) suggests a similar process for separating paraxylenes from mixtures of C.sub.8 -aromatics using a pyridine treated zeolite. In this instance, however, the zeolite is NaY instead of LiX or NaX. As in the above-noted process, the zeolite adsorbs the other C.sub.8 -aromatics to the substantial exclusion of paraxylene.
U.S. patent application Ser. No. 426,242 by Hulme and Barthomeuf teaches the use of various adjuncts including water, alcohols, ammonia and pyrrole adsorbents used to recover ethylbenzene from KY zeolites. The feedstream treated by these zeolites is taught to be substantially depleted in paraxylene.